Innate immune cells such as macrophages and neutrophils can promote or hinder tumor growth, but it is not known how these opposing activities are regulated in the tumor microenvironment. The major goal of this grant is to elucidate the mechanisms by which innate cells are activated to destroy tumor cells. To study how tumors may activate innate immunity, we have produced matched sets of tumor cell lines that are rejected by the immune system (termed regressors) or grow progressively (termed progressors) when transplanted into syngeneic, na?ve, wild-type mice. We have performed comparative studies to identify key immune system components that are enriched in tumors that undergo rejection compared to tumors that grow progressively. Using flow cytometry, immunohistochemistry, and microarray analysis, we have identified activated macrophages, neutrophils, nitric oxide, and the cytokine IL-17D as participants in tumor rejection. Our specific aims seek to understand the mechanism by which these cells and molecules interact to effect tumor rejection. The principle hypothesis states that IL-17D produced in the tumor microenvironment recruits neutrophils and activates macrophages to eliminate tumor cells via nitric oxide production. Specific aim 1 will study the role of IL-17D in shaping innate and adaptive anti-tumor responses. We will modulate IL-17D levels in tumor cells and observe the effect on tumor growth and immune cell infiltration. Specific aim 2 will explore how macrophages can promote tumor rejection by measuring macrophage tumoricidal activity in vitro and in vivo. We will compare macrophages isolated from regressor versus progressor tumors. Specific aim 3 will examine neutrophil effector pathways that are elicited by regressor tumors. These studies will enhance the efficacy of cytokine-based anti-tumor immunotherapeutic approaches while providing inroads into how innate immune cells can eliminate developing tumors.